He was actually the person that discovered radioactivity. He did so when studying phosphorescence, and discovered that uranium salts (that are phosphorescent too), also exposed a photographic plate even when they not had been exposed to light, and even through black paper. You can clearly argue about the share of the price, and he was also clearly doing the discovery by luck.

The Curies, and especially Marie Curie was the ones that sorted out the nature of radioactivity and discovered two new elements in the process, but the discovery of radioactivity itself was quite important, so that he received the Nobel prize was not really undeserved.